1. Field of the Invention
This invention pertains to raise boring. More particularly, it pertains to an improved method of raise boring, and to an improved raise bore drill string.
2. Review of the Prior Art and the Problems Thereof
Raise boring is commonly encountered in mining. A raise is a vertical shaft extending between different levels in a mine, or from a room or passage in a mine to the exterior of the mine, as for a ventilation shaft, for example. In boring a raise, a pilot hole is first drilled downwardly along the desired line from the top of the desired raise to the lower end thereof. Personnel at the bottom end of the pilot hole remove the pilot hole bit from the drill string (composed of serially connected lengths or "joints" of drill collars or heavy-wall drill pipe) and connect to the lower end of the drill string a raise bore bit. The raise bore bit is arranged to cut upwardly into the formation around the pilot hole as the drill string is simultaneously rotated and raised. The pilot hole bit may be arranged to form a hole of, say, 97/8 inches diameter as it proceeds downwardly through the formation. The raise bore bit, on the other hand, may be arranged to cut a hole of, say, 48 inches diameter as it proceeds upwardly through the formation. Bits, procedures, and equipment for performing these operations are known for raises in excess of 12 feet in diameter.
A procedure has been developed to commercially develop underground oil shale deposits in situ. This procedure involves sophisticated mining techniques and one desirable embodiment involves the creation of large numbers of raises in the oil shale deposit as a preliminary to recovery of the shale oil from the deposit. In order that such an approach to shale oil recovery may be practiced economically, it is important that all mining-like operations preliminary to the actual recovery stage, including creation of the many necessary raises, be performed as efficiently and as economically as possible. It is at this point that the limitations of present raise boring techniques and equipment begin to present problems of efficiency, to which problems this invention is addressed.
It should be noted that while this invention and its positive economic impact are perhaps best illustrated in the context of in situ oil shale recovery operations, the invention has utility in all aspects of raise bore drilling wherever encountered.
The problems and limitations of existing raise boring procedures to which this invention is addressed center around the difficulties presented in removing from the drill string a length or lengths thereof, as is required from time to time, as the raise proceeds upwardly and the drill string emerges progressively from the upper end of the pilot hole. These difficulties are best illuminated by a comparison of conventional down-hole drilling procedures with up-hole (raise bore) drilling procedures.
Down-hole drilling procedures followed in forming the pilot hole may rely on either a power swivel or a rotary table to apply torque to a conventional drill string composed of lengths of hollow, round cross-sectional drill pipe serially connected by conventional coaxial threaded couplings. A power swivel is essentially a motor (electric, hydraulic or pneumatic) having a hollow shaft connected to the upper end of the drill string and through which a circulating fluid (air or drilling mud) is introduced into the drill string for flow out of the drill bit and back up the drilled hole to cool the bit and to clear the bit and the hole of cuttings generated by operation of the drill bit. The power swivel is suspended in a suitable derrick positioned over the hole. When the hole has progressed downwardly an amount equal to the length of a single piece of drill pipe (a single piece commonly being called a "joint") or by an amount equal to the length of a "stand" (a group of two or three preassembled joints), rotation of the drill string is discontinued. The drill string is then secured in the hole by inserting suitable wedging chocks (called "slips") into the hole around the string below the connection of the swivel to the string. The swivel is disconnected from the string, raised in the derrick, and a new joint or stand is connected between the string and the swivel. The slips are then released and drilling is resumed.
Where a rotary table is used in down-hole drilling, as is common in the oil and gas industry, the swivel is passive and serves principally as a means for introducing circulating fluid into the drill string as it is rotated by the rotary table. A non-round, usually hexagonal or square length of special pipe, called a "kelly", is connected between the swivel and the upper end of the drill string. The kelly is, in effect, a long spline which cooperates with rollers in a kelly bushing carried in the rotatably driven annular member of the rotary table which is located in the base of the derrick. The kelly bushing rollers cooperate with the hexagonal or square configuration of the kelly to apply torque to the kelly and to accommodate axial motion of the kelly as the hole proceeds downwardly. The kelly has a length greater than the longest joint or stand used to make up the drill string. When the hole has increased in depth by an amount about equal to the length of the kelly, the drill string is raised through the kelly bushing until the coupling between the kelly and the drill string is above the rotary table, and slips are inserted into the bushing to prevent the string from falling back into the hole. The kelly is unscrewed from the drill string, a new joint or stand is added to the upper end of the drill string, and the extended string is lowered back into the hole until the upper end thereof is just above the rotary table, at which point the slips are again applied to secure the drill string. The kelly is then reconnected to the upper end of the drill string as extended, the slips are removed, and the string is lowered back into the hole so that the kelly reengages the kelly bushing. Drilling is then resumed until it is necessary to add another joint or stand to the drill string.
During down-hole drilling, removal of a joint or stand is often necessary, as where the drill string must be removed from the hole to change drill bits. This is no problem because, whether a power swivel or rotary table and kelly are used, the coupling between the drill string and the joint or stand thereof to be removed is readily made accessible above the top of the hole merely by raising the drill string the required distance out of the hole. In raise boring, however, the large diameter upwardly-cutting raise bore bit prevents the drill string from being raised in the hole except as the bit itself cuts upwardly. This fact, coupled with the fact that raise boring can be done through a rotary table rather than by use of a power swivel, means that the drill string cannot be raised to expose the coupling between the kelly and the drill string above the rotary table. It is for this reason that removal of joints or stands from a drill string during raise boring is a difficult, often hazardous, and time consuming procedure.
The pilot hole for a raise bore is often drilled using a power swivel rig. This is satisfactory since the pilot hole is small in diameter and the drill bit is cutting downwardly so that the proper drill bit loads can be established by weights added to the drill string as needed. The power swivel is not required to carry large axial loads, only to generate torque at moderate levels compared to the levels of torque required to operate a raise bore bit. In raise boring, however, the drill string torque levels are very high due to the size of the raise bore bit, and the drill string is maintained under considerable tension to establish the proper axial load on the raise bit. Power swivels are not well suited to the generation of high levels of torque or to prolonged application of large axial loads, unless the power swivel is very large and heavy, and quite expensive. Rotary table drilling rigs, on the other hand, do not carry any axial loads during actual drilling (axial loads are carried by the travelling block in the rotary table rig) and can economically apply large torque loads to a drill string via a kelly. It is for these reasons that rotary table rigs, rather than power swivel rigs, are used during raise boring operations.
From the foregoing, it is seen that existing raise bore drilling techniques and equipment present problems in the area of removal from the drill string of joints or stands thereof no longer needed as the raise proceeds upwardly. A need exists for improved raise bore drilling techniques and equipment which overcome these problems.